Air power wind turbines or wind mills are complex energy conversion systems that harnesses wind as a power source for the production of electricity, which are today increasingly attracting more interest as an alternative renewable source of energy.
Said wind-electric turbine generators, also known as wind turbines, are basically composed of a rotor consisting of one or more blades that convert wind energy into rotational/mechanical energy. As the air flows past the rotor of a wind turbine, the rotor spins and drives the shaft of an electric generator to produce electricity. As the nature of aerodynamics generally limits the speed of rotor to levels below that required by standard generators, a speed increasing gearbox is generally required for operating economically the turbine.
Thus, a gear-box is typically placed between the rotor of the wind turbine blade(s) and the rotor(s) of generator(s). More specifically, the gear-box connects a low-speed shaft turned by the wind turbine blade(s) rotor at about 10 to 30 rotations per minute (rpm), generally about 15-20 rpm to one or more than one high speed shaft that drives the generator to increase the rotational speed up to about 1000 to 2000 rpm, the rotational speed required by most generators to produce electricity.
The evidently high torque can generate huge stress on the gears and bearings in the geared wind turbine. Wind turbine oils are thus desired that will enhance the fatigue life of gears in the wind turbines.
Lubricants in said speed-increasing units or gearboxes have to fulfil several different roles. They must work at higher operating loads while helping in reducing temperatures in the gearboxes. They need to avoid fatigue-related damages (e.g. pitting) and wear (adhesion, abrasion, polishing and scuffing) on the gears, while also remaining fitter-friendly (no leaks), non-foaming, water-resistant, and harmless to operators.
Also, inasmuch as lubricants in wind turbines gearboxes are often subjected to prolonged periods of use between any maintenance and service intervals, a long lasting lubricant stability is required, so as to provide outstanding service performance over lengthy durations of time.
Finally, wind turbines can be located all over the world, on mountain tops, off-shore or along coastlines, in deserts: in addition to longevity issues, said lubricants must also be able to withstand a variety of environmental conditions, including temperature extremes and moisture, in addition to being able to resist oxidation and prevent corrosion.
Lubricant compositions have already been proposed in the past, at least partially fulfilling above mentioned requirements.
Thus, US 2005090410 (ETHYL CORPORATION) 28 Apr. 2005 discloses lubricant compositions suitable as gear oils for wind turbine comprising an additive concentrate composed of extreme pressure additives, load capacity enhancers and friction modifying compounds to be used in combination with a base oil either from natural sources (hydrocarbon oils of lubricating viscosity derived from petroleum, tar sands, coal, shale, and so forth, as well as natural oils such as rapeseed oil) or synthetic (e.g. poly-[alpha]-olefin oils, hydrogenated polyolefins, alkylated aromatics, polybutenes, alkyl esters of dicarboxylic esters, complex esters of dicarboxylic esters, polyol esters, polyglycols, polyphenyl ethers, alkyl esters of carbonic or phosphoric acids, polysilicones, fluorohydrocarbon oils).
Also, ERRICHELLO, Robert, et al. Oil Cleanliness in Wind Turbine Gearboxes. Machinery Lubrication Magazine. July 2002. discloses lubricating wind mill gearboxes with ester-based lubricants and mineral oils with antiscuff additives.
Similarly, a recent press release from Kluber Lubrication GB, published on Engineeringtalk (www.engineeringtalk.com) on Dec. 6, 2006, has proposed synthetic gear oils for wind turbines based on polyalphaolefins, polyglycol or rapidly biodegradable esters.
Nevertheless, lubricating oils of the prior art fail to provide for high thermal resistance and resistance to oxidation and exhibit changes in viscosity at rising or falling temperature which are inappropriate for complying with both cold start-up and steady state operations of the gear box of a wind turbine.
In particular, the lubricant of the prior art undergo significant viscosity increase at low temperature and are characterized by pour points close to temperatures which can be at least occasionally encountered during a wind mill start-up either after a shutdown or idling or non-rotation after low or no-wind periods during the year.
Also, lubricants of the prior art possess hazardous flammability properties, so that their use, in particular at high temperatures as occasionally encountered in wind turbine gear boxes as a function of wind power and of outdoor temperature (e.g. in sunny summer times), might be dangerous or locally expose the material to temperatures higher than their flash point.
In addition, lubricants of the prior art, in view of their flash point, evaporation behaviour (risk of evaporative losses) and thermal stability, often represent the actual limiting element for setting upper operating temperature boundaries, requiring special actions and measures, other parts of the gear box assembly being designed to possibly withstand higher temperatures.
Finally, lubricants of the prior art are highly sensitive to contamination from aqueous pollutants, e.g. of brine or other moisturized contaminants. Actually, as well known in the art, lubricants contamination is a dangerous source of failures of gear boxes of wind turbines, and contamination from aqueous pollutants especially in off-shore wind turbine fields is quite recurrent. Now, lubricants of the prior art do not offer suitable intrinsic protection against aqueous pollutants and purification processes for removal of said pollutants are burdensome. Synthetic ester lubricants have generally poor hydrolytic stability, that is to say that in contact with water might deteriorate through split back into an alcohol and an organic acid, totally compromising lubricating properties. Polyalkyleneglycol (PAG) oils might even be water miscible, so that they can be used only where condensate or water ingress is minimal.
The need was thus felt to provide a method for lubricating gear boxes of a wind turbine wherein the lubricant offer a wider operating temperature window, increased thermal and oxidative stability, non-flammability behaviour and increased resistance to aqueous pollutants so as to enable extended service life, while still providing adequate friction coefficients, wear protection, scuff resistance and load capacity.